1. Field of the Invention
The present invention relates to a stereo microphone having stable directional frequency response in a high-frequency range without generation of intrinsic noise.
2. Related Background Art
As disclosed in Japanese Unexamined Patent Application Publication No. H06-303691, a stereo microphone includes two microphone units, which output signals from left and right channels respectively.
Such a stereo microphone has, for example, two bidirectional microphone units, one of which has a directional axis directed at an angle of −45° to the left relative to the front of the microphone and the other has a directional axis directed at an angle of +45° to the right. The stereo microphone employs a Blumlein array for sound collection. The bidirectional microphone units configuring the Blumlein array include electrostatic condenser microphone units and electrodynamic ribbon microphone units.
The two microphone units included in the stereo microphone are generally disposed such that their directional axes are provided in the same horizontal plane. For the bidirectional microphone units used in the Blumlein array, however, it is not preferred in view of the performance that the left and right channel units be adjacently disposed since the properties are adversely affected unless a proximate construction is designed front/back symmetrically relative to the sound center. Thus, the two bidirectional units of the Blumlein stereo microphone are vertically stacked.
In such a Blumlein stereo microphone, the directional axes of the two microphone units are present in different horizontal planes, which configuration is not preferred for stereo sound collection. Since the directional axes of the two vertically stacked microphone units do not reside in the same horizontal plane, vertical (upper and lower) imbalance of a sound source relative to a proximate sound source, in particular, is picked up separately in the upper and lower microphone units, thus resulting in horizontally (left and right) unbalanced output.
It is thus desired for the stereo microphone to have compatibility between prevention of impact on acoustic properties due to a proximate construction and satisfactory stereo sound collection by disposing microphone units in the same plane.
The Blumlein stereo microphone may include bidirectional condenser microphone units. The bidirectional microphone units each have sound terminals in the front and back. A diaphragm vibrates in response to a sound pressure gradient determined by the distance between the front and back sound terminals. In order to achieve desired sensitivity, it is necessary to easily generate the sound pressure gradient, thus requiring a certain distance between the sound terminals. A long distance between the sound terminals, however, lowers a high-frequency sound collection limit.
In the case of using the bidirectional condenser microphone units, each of which generally includes a circular diaphragm, a large diameter of the unit is required for higher sensitivity.
One method of stereo sound collection is to combine two unidirectional condenser microphone units at a 180° direction to each other such that output sound signals are subtracted to achieve bidirectivity. Disposing the bidirectional sound collection axes configured as above in the same horizontal plane can solve the problem caused by vertical arrangement of the bidirectional microphone units described above, thus allowing stereo sound collection similar to the case of using the bidirectional condenser microphone units.
A similar configuration is provided in a four-channel one-point pickup microphone. Such a stereo microphone includes four unidirectional condenser microphone units disposed at different directions by 90° in the horizontal plane and back sound terminals sonically combined. Subtracting sound signals of the two unidirectional condenser microphone units disposed in the 180° direction provides bidirectivity.
Since the back sound terminals of the two unidirectional condenser microphone units disposed in the 180° direction are sonically combined, the distance between the sound terminals is inevitably long. Such a long distance between the sound terminals lowers the high-frequency sound collection limit, similar to the case of using the two bidirectional condenser microphone units described above.
With reference to FIGS. 4A to 4C, in a layout, four circular unidirectional condenser microphone units 10a, 10b, 10c, and 10d are disposed in the same horizontal plane such that external diameter portions thereof are in contact with each other by rotating directional axes of adjacent condenser microphone units 10 by 90°. A microphone having such a unit layout is referred to as a “four-channel one-point microphone,” which collects sounds from four directions and converts the sounds separately into sound signals for output.
In the unit layout shown in FIGS. 4A to 4C, collaboration of the two diagonally-positioned condenser microphone units 10a and 10c or 10b and 10d, each having directional axes disposed at 180° to each other, provides a pair of bidirectional microphone units. The two pairs of bidirectional microphone units are disposed such that the directional axes are disposed at 90° to each other, and thereby a stereo microphone is provided.
In this case, a distance W2 between sound terminals of the pair of microphone units is defined by the diameter of each of the condenser microphone units 10. In the condenser microphone unit, as the effective capacitance increases between a diaphragm 101 and a fixed electrode 102, the sensitivity increases while the effective noise decreases. In order to increase the effective capacitance, it is necessary to increase the area S2 of the diaphragm 101 of the condenser microphone unit 10.
In order to increase the area S2 of the diaphragm 101 of the condenser microphone unit 10, it is necessary to increase the diameters of the diaphragm 101 and the condenser microphone unit 10. The increased diameters thereof, however, lead to a large distance W2 between the sound terminals of the pair of microphone units, thus reducing the high-frequency sound collection limit, as shown in FIG. 5.